The present invention relates to a boiling water type reactor, and is more particularly concerned with a fuel assembly capable of carrying out a spectrum shift operation by controlling a flow resistance in a water cross or water rod arranged in the fuel assembly and also concerned with a reactor core equipped with the fuel assembly.
One conventional example of a fuel assembly which is charged in a reactor core of a boiling water type reactor (BWR) is shown in FIG. 21.
Referring to FIG. 21, a fuel assembly 1 is composed of a cylindrical, square in cross section, channel box 2 and a bundle of fuel rods 3. The fuel rod bundle 3 includes a plurality of fuel rods 11 arranged in square lattice form of 8 rows and 8 columns, for example, and a water rod 5 arranged at a central portion of the arrangement of the fuel rods 11. The water rod 5 has a diameter larger than that of the fuel rod 11. These fuel rods 11 and water rod 5 are supported together with a plurality of spacers disposed with spaces along the axial direction of the fuel assembly. An upper end plug 46 and a lower end plug 47 are mounted at the upper and lower end portions of these fuel rods 11 and the water rod 5, respectively. The upper end plug 46 is secured to an upper tie plate 12 provided with a handle 8 and the lower end plug 47 is also secured to a lower tie plate 13 provided with a coolant guide inlet 15.
A core water functioning as a moderator and a coolant flows into the fuel assembly as shown by arrows through the guide inlet 15 of the lower tie plate in a state that the core water rises upward through gaps between the respective fuel rods 11, and during the flowing, the core water removes a heat from the respective fuel rods 11 through heat exchanging function, thus the core water being finally formed at the upper portion of the core into steam and liquid flows.
The water rod 5 is provided with an opening 5a formed at the lower end thereof, and during this flow of the core water, the core water flows into the water rod 5 through this opening 5a, rises gently upward along the axial direction thereof and finally flows outward through a discharge port 5b into the upper portion of the fuel rods 11. The core water flowing in the water rod 5 mainly acts as the moderator and is mixed with the mixture of the steam and liquid flows at the upper portion of the core.
FIG. 22 shows an example in which a water cross 4 having a cruciform flow passage in cross section is arranged in place of the water rod 5 of FIG. 21. The water cross 4 has a coolant introducing port, not shown, at its lower end portion and has an upper end portion opened in cross shape. In FIG. 22, reference numeral 6 denotes a control rod. The structure of the control rod 6 is shown in FIG. 23, and referring to FIG. 23, the control rod 6 is composed of a sheath 117 in which a poison tube 118 is accommodated and which has an end secured to a central structural member 119. The sheath 117 also has an upper end to which is secured a handle 115 provided with a guide roller 116. To the lower end of the sheath 117 is secured a lower skirt 123, which is provided with a handle 120 for carrying out a separation from a control rod driving mechanism, not shown, a speed limiter 121 at the lower portion and a control rod driving mechanism socket 122.
The conventional BWR, as disclosed, for example, in the Japanese Patent Laid-open Publication No. 54-121389, has a core in which is arranged a fuel assembly provided with a water rod in which only the coolant flows for facilitating the moderation of neutrons. In the use of such water rod, reactivity of the core is made high as the number of the hydrogen atoms with respect to uranium atoms increases under the reactor running condition of the conventional BWR, thus enabling effective use of a nuclear fuel material charged in the core.
However, in order to further increase the effective use of the fuel material, it is better to change the number of the hydrogen atoms in the core in accordance with the burnup of the fuel material.
Advantages attained by changing the number of the hydrogen atoms in accordance with the burnup of the fuel material will be described hereunder.
FIG. 9 is a graph showing a relationship between burnup (axis of abscissa) and infinite multiplication factor (axis of ordinate) with respect to a typical example of the fuel assembly charged in the BWR. In FIG. 9, solid and broken lines both represent the same fuel assembly, but the broken line represents a case in which the burning is performed with constant void fraction (40%) in the coolant flow passage in the fuel assembly and, while, the solid line represents a case in which the reactor is operated initially with a high void fraction of 50% and with a reduced void fraction of 30% on the way of the operation. As can be understood from the graph of FIG. 9, more improved multiplication factor can be obtained at the final stage of the fuel life by burning initially with high void fraction and then reducing the same, and that is, a higher discharge exposure can be obtained.
This is because that the mean speed of the neutron becomes large and the neutron is easily absorbed by uranium 238 in the case of the high void fraction and the reduced number of the hydrogen atoms, i.e. small ratio of the number of the hydrogen atoms with respect to the number of the uranium atoms. A fuel material utilized for the BWR includes several % of uranium 235 and large % of uranium 238, thus almost uranium 238. In these uraniums, only the uranium 235 mainly absorbs the neutron and causesfission the uranium 238 hardly causes nuclear fission, and accordingly, as the uranium 235 reduces in its amount by the burning, the reactivity is lowered.
However, the uranium 238 is transformed to plutonium 239 by the absorption of the neutron having high energy caused by the fission. The plutonium 239 also causes fission by absorbing slowing-down thermal neutron like as the uranium 235. The higher the void fraction is, the higher the neutron energy is and, hence, the larger is the percentage for transforming the uranium 238 to the plutonium 239, whereby the fission of the uranium 235 and the plutonium 239 can be suppressed. Accordingly, the higher the void fraction is, the slower is the reduction of the whole quantity of the uranium 235 and the plutonium 239.
It is however noted that when the void fraction is high, the absolute value of the reactivity is low, and for this reason, when the high void fraction is kept as it is, the reactivity easily or speedily reaches its minimum level for keeping criticality in comparison with the low void fraction. Then, by lowering the void fraction at that time, the slowing-down effect of the neutron is increased and the fission of the uranium 235 and the plutonium 239 is thereby increased in comparison with the case of the constant void fraction, thus increasing the reactivity. Accordingly, the fissile material contained in the fuel material can be burned more longer till the reactivity reaches the minimum level necessary for the criticality.
The above described technical facts are the principle for achieving the effective use of the fuel material by changing the void fraction in accordance with the burning of the fissile material, which is hence called a spectrum shift operation.
One method of changing the number of the hydrogen atoms in the core in accordance with the burning of the fuel material for such spectrum shift operation has been proposed in the "Large Width Spectrum Shift BRW Core Concenpt (1)", No. F15 presentation on 1998, 4/4-4/6, by "SHO-63 Aunual Meeting" of Atomic Energy Society of Japan and in the Japanese Patent Laid-open Publication No. 63-73187. In these publications, as shown in FIGS. 24 and 25, a fuel rod support member 14 acting as resisting member is disposed at a lower portion of a fuel assembly la and a water rod 9 is provided with an inner tube 35 having a coolant inlet port 42 opened at an area below the resisting member 14 and having inside coolant rising passage 40. The water rod 9 also has a coolant lowering passage 41 and a coolant discharge opening 43 at an area above the resisting member 14 on that coolant lowering passage 41. The coolant lowering passage 41 is connected to the coolant rising passage 40 at the opening 34. The water rod 9 further includes an outer tube 36 supported to the inner tube 35 by means of spacers 37 and plugged with an end plug 38 at its upper end opening.
In the fuel assembly of the structure described above, as shown in FIG. 26, when the flow rate of the coolant passing the core decreases, the pressure difference between the inlet and outlet portions of the water rod 9 is reduced and steam is hence filled up in the flow passage of the water rod 9. On the contrary, when the flow rate increases, that pressure difference is increased and the steam in the passage of the water rod 9 is extremely reduced. Accordingly, it becomes possible to widely change the average void fraction in the fuel assembly and the increasing of the reactivity at the end of the reactor running cycle. Namely, during the initial half reactor running cycle in which the coolant flow rate is throttled, the moderator density is made large at the core lower portion in which a liquid phase exists in the flow passage of the water rod, and also the moderator density is made small at the core upper portion in which a steam phase exists therein. Therefore, during the initial half reactor running cycle, fuel material located in the core lower portion is mainly burned and the uranium 238 is transformed into the plutonium 239 in the core upper portion. On the other hand, during the later half reactor running cycle, the plutonium 239 transformed from the uranium 238 in the core upper portion during the initial half running cycle is mainly burned, whereby the fuel efficiency of the fuel material can be enhanced due to the spectral shift effect.
However, with reference to the conventional fuel assembly of the characters described above, in order to largely change the average void fraction in the fuel assembly, it is necessary to control the pressure difference between the inlet and outlet portions of the water rod in response to the core flow rate. By the way, in the BWR, the coolant flow rate depends on the output power and the axial power distribution of the fuel assembly. Accordingly, as the power of the fuel assembly becomes large, the void quantity is made large and the coolant flow rate of the fuel assembly is reduced by the increasing of two phase pressure drop. Further, in the case of the same output power of the fuel assembly, when the axial power distribution has downward peak, the void quantity is made large and the coolant flow rate is reduced by the increasing of the two phase pressure drop. The variation of the coolant flow rate due to the power distribution has a wide range of 20%. As shown in FIG. 26, the average void fraction of the water rod largely changes in response to the minute change of the pressure difference between the inlet and outlet portions of the water rod. Accordingly, for example, even if the water rod of the fuel assembly is surely controlled to about 10% of the low void fraction in a case where the reactor is operated with 110% of the rated core flow, and even if the water rod is surely controlled to about 70% of the high void fraction in a case where the reactor is operated with 70% of the core rated flow, with respect to an intermediate core flow rate between 110% and 70%, there causes large dispersion of the void fraction in the water rod depending on the power of the fuel such as 10% and 70%. As this result, there causes large difference between a signal of nuclear monitoring instrumentation in the core and an evaluation result based on three dimensional nuclear-thermal-hydraulic calculation program in which the power of the fuel assembly is monitored and simulated through an online, which results in a defect of being disadvantageous to the evaluation of thermal limitation with high performance in the core (MCPR, MLHGR). Furthermore, for the conventional water rod, it is necessary to widely change the core flow rate in order to carry out the spectral shift operation in which the void fraction in the water rod is widely changed, and in view of the limitation such as MCPR, there provides a defect such that the spectral shift cannot be performed in a case where the coolant flow rate cannot be throttled.